The Prediction of Creep-Fatigue Using an Improved Ductility Exhaustion Method

Abstract The thermal mechanical cycling of high temperature components can result in creep-fatigue cycles in which the creep dwell has a wide variety of positions within the cycle. The effects of these different types of creep dwells have been investigated as part of the R5 development programme which has resulted in the modification of R5 Issue 3. The materials included in the R5 programme on intermediate dwells were cast 1CrMoV and cast ½CrMoV ferritic steels and three austenitic stainless steels; a Type 316H steel, a cast Type 304L and a Type 347 weld metal. In addition, existing data with dwells at maximum strain were investigated on Grade 9, Grade 91 and Type 321. The new method for the calculation of creep damage is referred to as the ‘stress modified’ ductility exhaustion, SMDE, approach and has been shown to give better predictions for the creep damage at failure in laboratory tests compared with both the traditional ductility exhaustion approach and the time fraction approach. In particular, the ‘stress modified’ ductility exhaustion approach gives significantly improved predictions of creep damage at failure for tests where the creep dwells are positioned at a stress below the peak in the cycle, (intermediate dwells) and for cycles with low strain ranges with peak dwells. Such cycles are typical of many high temperature components, which experience a thermal fatigue cycle followed by a creep dwell. Thus, it would be anticipated that the use of the ‘stress modified’ ductility exhaustion approach to assess real plant like cycle would lead to more realistic predicted life times when compared with the traditional R5 ductility exhaustion approach.